Calcium movements during E-C coupling in giant muscle fibers from the barnacle will be followed using the aequorin technique. The entry of extracellular calcium during the calcium spike will be determined by simultaneously measuring the calcium current with a voltage-clamp and calcium ion entry with aequorin. Normal calcium release from the sarcoplasmic reticulum together with normal extracellular calcium entry will be determined for the full range of depolarizing pulses and consequent contractile forces in both isometric and isotonic contractions. The relationship between calcium level and force will be investigated using a calcium-clamp to control the level of calcium activation. Biochemical measurements of the calcium binding constants of the barnacle contractile proteins will be used to further characterize the relationship between calcium and force.